About Ammonification & pH Levels

Ammonification Chemistry

• In agriculture, farmers inject ammonia into the soil to provide crops with nitrogen. Natural production of ammonia in soils happens by microbial conversion of proteins from decomposing organic matter. Proteins consist of chains of amino acids, which are made of carbon (C), hydrogen (H), oxygen (O) and nitrogen (N). The amino acid molecule can be divided into two parts, the R- or CHO-portion and the amine (NH2) group. The ammonification process removes the amine group from the amino acids, converting it to ammonia.

pH and the Conversion of Ammonia to Ammonium

• When the pH of the soil is less than 7.5, the ammonia molecule is rapidly converted to the ammonium ion. This occurs because of the relative abundance of hydrogen ions present in soil solution at that pH. When the pH is greater than 7.5, the conversion to ammonium to ammonia gas is favored, thus causing loss of nitrogen from the soil. If the pH is acidic, it can also decrease the amount of conversion (nitritification) of the ammonium ion to nitrate (NO3) by the microorganisms Nitrosomas and Nitrobacter. Nitrification stops at a pH of 6.0 or below.

Ammonification Microorganisms and pH

• Various species of bacteria, actinomycetes and many fungi can convert amines to ammonia. The bacterial species include Clostridium and heterotrophic species like Pseudomonas, Bacillus and Proteus. Streptomyces is an actinomycete that is known to facilitate ammonification. Generally speaking, the activity and growth of bacteria is inhibited at lower pH values. Actinomycetes and fungi are not affected by low pH as much. Thus, ammonification, if primarily facilitated in a certain soil by heterotrophic bacteria, would be less in soils with low pH values. Fungi are much less affected by pH values in their growth, but an effect of pH levels on ammonification abilities of soils has been found in research studies. Researchers speculate that aluminum ion toxicity to microorganisms at low soil pH levels may be the cause of reduced ammonification.

Forest Soils and Ammonification Ability Based on pH

• There is research evidence that low pH soils are associated with less ability to facilitate ammonification. A comparison was done of forest soils having a pH less than 4.0 with soils having a pH greater than 4.0. An assay for measuring the ability of soils to produce ammonia was developed using the protein arginine. Ammonia production from this protein was significantly higher in the soils with the higher pH. Also, a negative correlation was found between pH and ammonia production in the low-pH soils.